The present invention relates to an apparatus for printing graphic and alphanumeric information on a printing media; more particularly, the invention relates to an apparatus for printing on plastic disks adapted for holding prerecorded information which has been embedded in the disk by a recording process which is outside the scope of the present invention. Disks of the general type related to this invention are commonly referred to as "compact" disks, or "CD's." These disks are frequently utilized for recording high quality audio and video information and also for recording computer software of various types. The disks are characterized by a construction which is formed of a plurality of layers, including a reflective inner layer, and a relatively smooth top surface, which usually contains printed characters and graphic information describing the disk, the disk content, and the manufacturer. The method and apparatus for printing information on the top surface of the disk must be carefully controlled, for the reasons which are described in more detail herein.
Compact disks are standardized in two sizes and configurations, one having an overall diameter of 4.72 inches, a central hole of 0.59 inches, and a central region about the center hole of 1.50 inches in diameter, wherein no information is either printed or recorded. The other standard disk size is 3.5 inches in overall diameter, with a comparable central hole size and central region. In the case of disks for utilization in connection with computer processors, the recording formats and content are typically adapted to the particular generalized type of computer processor with which the disk is to operate. Some compact disks are recorded in such a way as to be usable with several different computer processor types; i.e., PC, Macintosh, etc. The particular recording formats and conventions are not relevant to the present invention; the present invention is concerned merely with the printing process for assuring a high quality permanent image imprinted on the top surface of the compact disk.
The printing process for printing information and graphics on the surface of plastic disks, particularly compact disks, has typically involved a silk screening printing process or a printer utilizing ink jet printing technology. It is frequently necessary to prepare the disk with a special surface coating in order to cause the printed material to adhere to the disk. These processes are useful in a controlled industrial situation wherein disks are produced in quantity and wherein a large number of disks will necessarily have the same recorded information stored within the disk and, therefore, will require the same information content to be imprinted on the disk surface.
There is another type of compact disk which has become available and which is known by the designation "CDR" which refers to a recordable compact disk. This type of disk is initially prepared in blank, unrecorded form and may be subsequently recorded with unique or custom data at a work site which is remote from the industrial plant where the disk was manufactured. There is a need for a printer which can be used for imprinting identifiable data relating to such disks for which only a relatively few number of copies may be prepared. This silk screening process is not readily adaptable for this type of application, and the ink jet printing technology is not readily applicable unless the printable surfaces are precoated with a layer which will accept the ink jet printed material. However, a thermal printing process could be adapted to this application if a suitable printer were devised for thermal printing.
Thermal printing processes may be generally subdivided into direct thermal transfer devices and indirect thermal transfer devices. A direct thermal transfer device is a printer which utilizes a line array of individually and selectively heatable pixels which may be applied directly against a treated surface so as to directly create an image on the surface from the heated pixels. The surface must be coated with a material which is optically responsive to heat so that the heated pixels will create a permanent image. An indirect thermal transfer device is a printer utilizing a line array of individually and selectively heatable pixels which may be applied against an intermediate medium to cause the intermediate medium to apply the image to the surface. The intermediate medium is typically a heat-sensitive ink ribbon which is positioned intermediate the thermal transfer pixel line array and the CD surface. This process is uniquely adaptable for printing on a CDR, providing that the printing force per unit area; i.e., the force per pixel, is kept at a constant level for all pixels across the line array which contact the CD surface.
In the context of the present invention, both the indirect thermal transfer printer and the direct thermal transfer printer are individually and collectively referred to under the designation "thermal printer"; and it is understood that the terminology "thermal printer" includes either or both types of printing technology. The print pixel line array typically consists of over 1,500 individual pixels arranged linearly along about a five-inch printing line. The pixels are individually heated, and all or a portion of the line array is applied against the surface of either a CD or an inking ribbon which passes between the line array and the CD. The application of a force against the line array and heat applied to the selected pixels causes a print impression onto the CD.
It is desirable that the pressure against the compact disk be uniformly applied during the printing process in order to insure the highest quality of printing onto the compact disk. However, it is difficult to apply uniform printing pressure along a line array of printing pixels when there is relative movement between the compact disk and the printhead, because the length of the compact disk segment beneath the printhead will vary as the relative movement progresses. Initially, only a tangential edge of the disk is positioned beneath the printhead, and the length of the printing segment increases as the disk continues to move relative to the printhead until the maximum disk diameter passes the printhead. Thereafter, the length of the printing segment decreases as the relative movement of the disk continues until finally the trailing tangential edge of the disk passes the printhead. It is apparent that the amount of force applied to the line print array must vary with the length of printing segment beneath the printhead at any given moment, in order to achieve uniform printing pressure over all printing segments.